Many recent researches have found that by equipping transmitters of wireless communication systems with large antenna arrays, huge array gain can be attained and high data rate can be achieved with very low transmission power. Such communication scheme, known as Massive-MIMO, is a promising technique for future wireless communication systems due to its high power efficiency. However, the benefits of Massive-MIMO are mainly attributed to the availability of channel state information (CSI) at the transmitter. Many MIMO techniques, including beamforming, channel inversion, adaptive power/bit allocations, and interference alignment, require the transmitter to possess certain spatial channel information. Hence, most of the relevant research works in this context has focused on time-division duplex (TDD) schemes as channel information can be acquired by the transmitter by exploiting channel reciprocity. Additional feedback mechanism is required for frequency-division duplex (FDD) systems, and apparently feedback overhead is a formidable challenge for Massive-MIMO due to the large number of antennas. In order to realize Massive-MIMO under FDD mode, a more efficient channel information feedback mechanism for Massive-MIMO should be designed.
Most of the prevalent MIMO communication standards, such as LTE-A and WiMAX, employ codebook-based approaches to reduce feedback burdens. A codebook which consists of multiple entries should be pre-configured and stored at both the transmitter and the receiver. Each index of the codebook maps to a quantized version of MIMO spatial channel information. Therefore, feedback volume can be tremendously reduced by merely sending the index of the codebook entry that provides the most appropriate representation of the current channel structure.
Codebook-based quantization approach may work properly for moderate MIMO systems with two to eight antennas. For Massive-MIMO with numerous antennas, however, the codebook size has to be expanded extensively to capture all prospective spatial channel structures. This makes codebook design much more difficult. Apparently, expansion of codebook size also leads to heavier feedback overhead. On the other hand, the intrinsic quantization errors of codebook-based approach may not be acceptable for massive MIMO-based green radio techniques that aim to save transmission power via precise beamforming.